This invention relates to a multilateration system for determining a position of an object.
Multilateration systems are used to provide, in particular, the position of aircraft in flight or on airport runways. A signal transmitted by a transmitter on the aircraft is received by a number of receiver stations at known locations. The signal is transmitted by a 1090 MHz Secondary Surveillance Radar (SSR) transponder and is one of a number of known code types or formats Mode A/C and mode S. By comparing the time of arrival of the signal at each of the receiver stations and with the knowledge of their locations it is possible to calculate the position of the aircraft at the time of transmission. Such a system and a multilateration technique is described in patent GB2250154.
It will be appreciated that in order to accurately determine the position, it is necessary to cater for variations in apparent path length between parts of the system. This is in order that meaningful difference in time of arrival values for the signal can be derived. This can give rise to a factor called group time delay. Aging of components, for example, may cause a variation in the group time delay which may require servicing of parts of the system or other remedial action or correction.
The present invention arose in an attempt to determine which receiver in a group of receivers in multilateration system was experiencing a significant group time delay.
According to the invention there is provided apparatus for identifying which receiver in a plurality of receivers in known positions in a multilateration system is experiencing a significant group time delay comprising for in a first and subsequent pass: means to determine from a first group of receivers a detected position of a signal source; means to determine from the detected position and the known positions of a receiver or receivers of a second group predicted difference in time of arrival values for the signal for the second group; means to compare the predicted difference in time of arrival value with an actual difference in time of arrival value at the second group to derive a difference in time of arrival difference; means to derive a variation in successive difference in time of arrival differences; and means to determine when there is a significant variation in the difference in time of arrival.
Preferably, means are provided to allocate receivers to the first and second group. In the preferred embodiment the receivers are allocated such that all the receivers participate in the first group and also the second group.
The inventors have determined that when the lowest variation is experienced then the receiver in the second group is that having the greatest offset error. This because the first group is utilised to provide the position information which does not involve the use of the poorly performing receiver. This receiver may be adjusted or serviced as required, or the error may be compensated within the processing.
The invention also provides a method.